Metrology targets are designed to reflect device production accuracy in optical measurements of the targets. Imaging targets that comprise periodical gratings with large pitches (e.g., 1500-2500 nm) usually provide good contrast but are not process compatible and have a large pattern placement error (PPE, see below), which can reach a few nanometers. Scatterometry targets have smaller pitches (e.g., 500-800 nm) which are still an order of magnitude larger than device pitches. The gap between design rule pitches and SCOL target scales is large and PPE can be as large as in the case of imaging targets.
Pattern placement error (PPE) of the target pattern placement relative to the device pattern placement results from the current limitation of unachievable on-device overlay (OVL) metrology due the fact that the design rule pitches are unresolved by current imaging and scatterometry optical tools. Instead of on-device measurements, overlay measurements are performed on specially designed “proxy” metrology targets which have typical scales (pitches) that are larger than hundreds of nanometers (the gap between device pitch (<90 nm) and the pitch of “proxy” target increases with time). This difference in target and device scales manifests itself in pattern placement error (PPE). For example, due to scanner asymmetric aberrations, the PPE between devices and “proxy” targets may be as large as 1-2 nanometers. Accordingly, the measured OVL values should be corrected on the values of placement errors for resist and process layers which depend on a specific pair of scanners used for printing these layers and field position of the target. Such correction is an extremely difficult logistic problem which requires developing of alternative approaches for significant PPE reduction. Current metrology targets may provide good contrast and process compatibility but have a large pattern placement error, while suggested device-like targets, such as disclosed in U.S. Patent Publication No. 20110028004, which is incorporated herein by reference in its entirety, may satisfy the requirements for small PPE and process compatibility but do not provide a good contrast for OVL measurement.
U.S. Pat. No. 8,913,237, which is incorporated herein by reference in its entirety, discloses overlay targets comprising at least a plurality of a plurality of first grating structures having a course pitch that is resolvable by an inspection tool and a plurality of second grating structures positioned relative to the first grating structures. The second grating structures have a fine pitch that is smaller than the coarse pitch, and the first and second gratings have feature dimensions that all comply with a predefined design rules specification.
FIGS. 1A-1C are examples of prior art device-like targets 90 with corresponding prior art diffraction patterns. FIG. 1A schematically illustrates prior art device-like target 90 having a coarse pitch Pc and a fine pitch Pf (e.g., Pc=900 nm, Pf=90 nm), with varying widths of the coarse pitched elements, resulting in a produced target 93 at the coarse pitch that yields a diffraction pattern 96 (produced target 93 is illustrated as having more elements than target design 90 yet demonstrates the same principle). Such device-like targets have a small pattern placement error and are process compatible, but they do not provide enough contrast for overlay metrology measurements. FIGS. 1B and 1C schematically illustrates prior art device-like target designs 90 having perpendicular structures 91, 95 which are process compatible and improves significantly target contrast with respect to prior art device-like targets as illustrated in FIG. 1A, but do not solve the PPE problem. Simulations show that the PPE is about 3 nm nanometers for 50 mλ amplitude of scanner asymmetric aberrations. A seen in the corresponding prior art diffraction pattern 96, the amplitudes of diffraction orders corresponding to the coarse pitch are not small and the placement of the printed pattern differs from placement of devices row.